Groovy - List unique(boolean mutate, Closure closure) method

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Description

Groovy List unique(boolean mutate, Closure closure) method modifies the list by removing any duplicates. If mutate is true, then original list is modified as well. Duplicates are removed using Groovy's default number-aware comparator. Closure is used to determine unique items. If single argument is passed to closure, then it should return a value used for comparison. In case of two arguments are passed, then closure should return an int value where 0 indicates items are not unique.

Syntax

public List unique(boolean mutate, Closure closure)

Parameters

• mutate − if true, then original list is modified as well.

• closure − a closure to check uniqueness of elements.

Return Value

A list of unique elements of the list.

Example - Getting Unique elements from a List of Integers

Following is an example of the usage of this method −

main.groovy

def lst = [11, 12, 13, 10, 15, 11, 10, 12, 15]

// getting list of unique and even elements 
uniqueList = lst.unique(false){ it % 2}
println(uniqueList)

// getting list of unique elements 
uniqueList = lst.unique(false){ a, b -> a <=> b}
println(uniqueList)

Output

When we run the above program, we will get the following result −

[11, 12]
[11, 12, 13, 10, 15]

Example - Getting Unique elements from a List of Strings

Following is an example of the usage of this method −

main.groovy

def lst = ["Apple","Mango","Orange","Papaya","Apple", "Apple"]

// getting list of unique and even elements 
uniqueList = lst.unique(false){ it.size() % 2}
println(uniqueList)

// getting list of unique elements 
uniqueList = lst.unique(false){ a, b -> a <=> b}
println(uniqueList)

Output

When we run the above program, we will get the following result −

[Apple, Orange]
[Apple, Mango, Orange, Papaya]

Example - Getting Unique elements from a List of Objects

Following is an example of the usage of this method −

main.groovy

def lst = [new Student(1, "Julie"),new Student(2, "Robert"),new Student(1, "Julie")]

// getting list of unique and even elements 
uniqueList = lst.unique(false){ it.getRollNo() % 2}
println(uniqueList)

// getting list of unique elements 
uniqueList = lst.unique(false){ a, b -> a.getName() <=> b.getName()}
println(uniqueList)

class Student implements Comparable {
   int rollNo;
   String name;

   Student(int rollNo, String name){
      this.rollNo = rollNo;
      this.name = name;
   }

   @Override
   public boolean equals(Object obj) {
      Student s = (Student)obj;
      return this.rollNo == s.rollNo && this.name.equalsIgnoreCase(s.name);
   }
   
   @Override
   public String toString() {
      return "[ " + this.rollNo + ", " + this.name + " ]";
   }
   
   public int compareTo(Object obj){
      Student s = (Student)obj;
      return this.rollNo - s.rollNo   
   }
}

Output

When we run the above program, we will get the following result −

[[ 1, Julie ], [ 2, Robert ]]
[[ 1, Julie ], [ 2, Robert ]]